Contains research objectives and summary of research on one research project.Joint Services Electronics Program (Contract DAAB07-76-C-1400

Joannopoulos, John D.

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III. SEMICONDUCTOR SURFACE STUDIESAcademic Research StaffProf. John D. JoannopoulosGraduate StudentsRobert B. LaughlinEugene MeleWilliam B. Pollard1. ELECTRONIC STRUCTURE OF HOMOPOLAR AND HETEROPOLARSEMICONDUCTING SURFACESJoint Services Electronics Program (Contract DAAB07-76-C-1400)John D. JoannopoulosWe are interested in a theoretical investigation of the electronic states at the sur-faces of Group IV, III-V, and II-VI semiconductors. Because of the lack of completeperiodicity when dealing with a " semi-infinite" surface system, we are faced withsevere obstacles in making a realistic theoretical investigation. Nevertheless, we haverecently constructed a theorem demonstrating that any semi-infinite surface systemthat can be described by a Hamiltonian of finite range can be reduced to an effective one-dimensional problem that can be solved with transfer matrix techniques.We have used this theorem with realistic tight-binding Hamiltonians to study theeffects of surface atom relaxations in the heteropolar compounds. Our results showthat the unfilled surface states are very sensitive to the nature of the Hamiltonian thatis used. Given a relaxation, these states will move out of the gap if the cation/anionp-like character of the bulk conduction bands is large enough. In this way the behaviorof states at the surface is directly related to properties of the bulk system. It is there-fore of paramount importance for theoretical models to describe certain features of thebulk system correctly.At present, we are investigating the effects of oxygen adsorption on the surface ofGaAs. The O interaction integrals can be obtained from studies of molecular 0 2 andGeO 2 . In this way we hope to determine where the oxygen is adsorbed, and whether it isadsorbed as O or 02 or in some type of mixed bridging configuration.PR No. 119

Semiconductor Surface Studies

Contains research objectives and summary of research on one research project.Joint Services Electronics Program (Contract DAAB07-75-C-1346

III. SEMICONDUCTOR SURFACE STUDIESAcademic Research StaffProf. John D. Joannopoulos1. ELECTRONIC STRUCTURE OF HOMOPOLAR AND HETEROPOLAR JSEPSEMICONDUCTING SURFACESJoint Services Electronics Program (Contract DAAB07-75-C-1346)John D. JoannopoulosOur objectives constitute a theoretical investigation of the electronic states at thesurfaces of Group IV, III-V, and II-VI semiconductors. Specifically, we have beenstudying the existence and character of surface states by examining the complete valencebands for unrelaxed and unreconstructed surfaces. Our approach has been to use simpletight-binding Hamiltonians to study two types of surface systems. The structure of thesesystems is that of a diamond or a zinc blende that is respectively semiinfinite or finitein one direction, and infinite and periodic in the two other directions. Preliminaryresults show the presence of new surface states in the antisymmetric gap of III-V andII-VI surfaces.The effects of relaxation, reconstruction, and adsorbates are also being studied.Surfaces with steplike features will be examined to determine their effects on the elec-tronic properties of the surface. JSEPPR No. 117

Contains reports on one research project.Joint Services Electronics Program (Contract DAAB07-75-C-1346

III. SEMICONDUCTOR SURFACE STUDIESAcademic Research StaffProf. John D. JoannopoulosGraduate StudentsEugene \leleA. ELECTRONIC STRUCTURE OF HOMOPOLAR AND JSEPHETEROPOLAR SEMICONDUCTING SURFACESJoint Services Electronics Program (Contract DAAB07-75-C-1346)John D. JoannopoulosWe are interested in theoretical investigation of the electronic states at the sur-faces of Group IV, III-V, and II-VI semiconductors. Because of the lack of completeperiodicity when dealing with a "semi-infinite" surface system, we are faced withsevere obstacles in making a realistic theoretical investigation. Our approach hasbeen to replace the ideal semi-infinite system with an infinite "slab" system. Withthe use of such a system, our results on GaAs predict new surface states in theantisymmetric gap of the (]10) surface. The nature, localization, and character ofthese states has been determined.We have also developed a new method, a Gaussian relaxation method, which makestractable the study of finite but very large systems. It is a local formalism based onan effective Green's function from which local density of states and charge densitiescan be obtained. With the use of realistic tight-binding Hamiltonians, preliminaryresults on vacancies in Ge have yielded densities of states and charge densitiescomparable to those for Si obtained by using self-consistent pseudo-potential techniques.The Gaussian relaxation method should be very useful for studying surface systemswith large surface reconstructions, for example, Si (7 X 7), Ge (2 x 8).Finally, we have constructed a theorem demonstrating that any semi-infinitesurface system that can be described by a Hamiltonian of finite range canbe reducedto an effective one-dimensional problem that can be solved with matrix transformtechniques. This theorem is being used to study relaxation effects in GaAs (110) andthe effects of 02. JSEPPR No. 118

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Semiconductor Surface Studies

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